Countersink & chamfer
Countersink diameter and depth, chamfer in X and Z, drill point length. Three calculators for the NC program, plus the countersink dimensions for countersunk and cap screws from M3 to M20.
Countersink
Enter countersink depth or countersink diameter, the other is calculated.
The countersink depth is measured from the edge of the hole. The plunge depth is how far a pointed countersink or NC spot drill must go below the surface to reach the countersink diameter. Clicking a screw size sets the clearance hole and countersink diameter to DIN 74 Form F.
Chamfer
Chamfer a × angle on a turned part. a is the length along the axis, the angle is measured from the axis.
The start Ø is the diameter at which the chamfer begins on the face. At 45° the radial dimension and the length along the axis are equal.
Drill point
How much deeper must you program so the hole has its full diameter at full depth?
Formulas at a glance
How to calculate countersink diameter
d hole, t countersink depth from the hole edge, α countersink angle. At 90° tan(α/2) equals 1, so the diameter grows by twice the depth: a 6.6 mm hole countersunk 3 mm deep gives Ø12.6.
Plunge depth of a pointed countersink
D is the required countersink diameter. At 90° the plunge depth is half the diameter: for Ø13.7 the tip must go 6.85 mm deep.
How to calculate a chamfer
a length along the axis, b radial dimension, c length of the chamfer face. A 2 × 30° chamfer is 1.155 mm deep radially and 2.309 mm long.
Drill point length
σ is the point angle. At 118° it is about 0.3 times the diameter, at 140° about 0.18 times. A Ø8.5 drill with 118° has a 2.554 mm point.
Countersinks for screws M3 to M20
Left: hexagon socket countersunk screws to ISO 10642 with the countersink to DIN 74 Form F. Right: socket head cap screws to ISO 4762 with the counterbore to DIN 974-1 series 1. Clearance holes are the medium series. All dimensions in mm.
Countersunk screw ISO 10642, 90° countersink
| Size | Hole | Head Ø | Sink Ø |
|---|---|---|---|
| M3 | 3.4 | 6.72 | 6.9 |
| M4 | 4.5 | 8.96 | 9.2 |
| M5 | 5.5 | 11.2 | 11.5 |
| M6 | 6.6 | 13.44 | 13.7 |
| M8 | 9 | 17.92 | 18.3 |
| M10 | 11 | 22.4 | 22.7 |
| M12 | 13.5 | 26.88 | 27.2 |
| M16 | 17.5 | 33.6 | 34 |
| M20 | 22 | 40.32 | 40.7 |
Cap screw ISO 4762
| Size | Hole | Head Ø | Head height | Bore Ø |
|---|---|---|---|---|
| M3 | 3.4 | 5.5 | 3 | 6.5 |
| M4 | 4.5 | 7 | 4 | 8 |
| M5 | 5.5 | 8.5 | 5 | 10 |
| M6 | 6.6 | 10 | 6 | 11 |
| M8 | 9 | 13 | 8 | 15 |
| M10 | 11 | 16 | 10 | 18 |
| M12 | 13.5 | 18 | 12 | 20 |
| M16 | 17.5 | 24 | 16 | 26 |
| M20 | 22 | 30 | 20 | 33 |
Which angle for what
Metric countersunk screws have a head angle of 90°, inch countersunk screws to US standards 82°. A 60° countersink is for deburring and chamfering, 120° is used for sheet-metal rivets. Machining an American drawing with a 90° countersink gives a screw that only seats on its rim.
For hexagon socket countersunk screws to ISO 10642 the countersink follows DIN 74 Form F. The head of these screws is larger than under the old DIN 7991. Many tables online still show the old, smaller dimensions. A hole countersunk to those is too small for a current screw, and the head stands proud.
From the shop floor: If the screw head has to sit flush, countersink a test piece first and check it with the actual screw. Screws from different manufacturers vary by a few tenths in head diameter, and the countersink itself is rarely perfectly pointed.
Frequently asked questions
How do you calculate countersink diameter?
Hole diameter plus twice the countersink depth times the tangent of half the countersink angle. At 90° the tangent equals 1, so the diameter grows by twice the depth. A 6.6 mm hole countersunk 3 mm deep gives Ø12.6.
What size is the countersink for an M6 countersunk screw?
For the M6 hexagon socket countersunk screw to ISO 10642 the countersink diameter to DIN 74 Form F is 13.7 mm at 90°, with a 6.6 mm clearance hole. The countersink depth from the hole edge is therefore about 3.55 mm. The head itself has a theoretical diameter of 13.44 mm.
How deep must a 90° countersink plunge?
With a pointed 90° countersink the plunge depth from the surface is half the countersink diameter. For Ø13.7 that is 6.85 mm. If the countersink has a flattened tip, that amount must be subtracted.
How do you calculate a chamfer on a turned part?
At 45° the length along the axis and the radial dimension are equal. A 1 × 45° chamfer on Ø30 starts at Ø28 on the face and runs 1 mm in Z. For other angles the radial dimension is the length times the tangent of the angle.
How long is the point of a drill?
Half the diameter divided by the tangent of half the point angle. At 118° that is about 0.3 times the diameter, at 140° about 0.18 times. A Ø8.5 drill with 118° has a 2.554 mm long point. That is how much deeper you must program if the full depth is required.
What counterbore does an M8 cap screw need?
The M8 socket head cap screw to ISO 4762 has a head 13 mm in diameter and 8 mm high. The counterbore to DIN 974-1 series 1 is 15 mm in diameter, the clearance hole 9 mm. The counterbore depth follows from head height plus allowance and any washer.
Software for manufacturers
We built these calculators ourselves. For companies we develop Agentority, an AI that runs on your own server and works with your ERP, inbox and warehouse. It drafts quotes from customer emails, flags delivery delays and books in delivery notes from a photo.
Request parts
Send a drawing or STEP file. We review the enquiry and pass it on to a vetted partner shop. We will get back to you within 24 hours.
Start request